This invention relates to a process for producing an alkylation reaction product from an isoparaffin and an olefin-acting agent, utilizing hydrogen fluoride catalysts. In one aspect, this invention relates to a process for reacting lower molecular weight isoparaffins with lower molecular weight olefins to produce higher molecular weight branched-chain hydrocarbons suitable for use in motor fuel. In another aspect, this invention relates to a method for reducing the alkylation catalyst inventory requirements in a hydrogen fluoride catalyzed alkylation process. In a further aspect, this invention relates to a method for using two hydrogen fluoride catalyst phases, differing in acid strength, in an isoparaffin-olefin alkylation operation.
Alkylation of isoparaffinic hydrocarbons, such as isobutane and isopentane, with olefinic hydrocarbons such as propylene, butylene and amylenes, or with other olefin-acting agents such as C.sub.3 -C.sub.5 alkyl halides, etc., using hydrogen fluoride as a catalyst is well known as a commercially important method for producing gasoline boiling range hydrocarbons. The C.sub.5 -C.sub.10 hydrocarbons typically produced in isoparaffin-olefin alkylation operations are termed "alkylate". Alkylate is particularly useful as a motor fuel blending stock. It possesses motor and research octane ratings high enough that it may be employed to improve the overall octane ratings of available gasoline pools to provide motor fuels which comply with the requirements of modern automobile motors. High octane alkylate blending components are particularly important in producing motor fuels of sufficiently high octane when it is desired to avoid use of alkyl lead compounds in the motor fuel. A continuing goal in the art is to provide an economically attractive hydrogen fluoride catalyzed alkylation process which provides an alkylate product having motor and research octane ratings which are higher than are attainable in conventional alkylation processes.
In commercial isoparaffin-olefin alkylation operations using hydrogen fluoride catalyst, generally, isobutane is the isoparaffin used and propylene, butylenes, amylenes, or a mixture of these olefins, are used as the olefin-acting agent. In conventional prior art operations, the isoparaffin, olefin-acting agent and hydrogen fluoride catalyst are first contacted and thoroughly admixed in an alkylation reactor, forming a reaction mixture, or emulsion. After a relatively short time, reaction of the alkylating agent is substantially complete and the reaction mixture is withdrawn from the alkylation reactor and is allowed to settle by gravity into immiscible hydrocarbon and catalyst phases in a settling vessel. The hydrogen fluoride catalyst phase thus separated is returned directly to the alkylation reactor for further catalytic use. The hydrocarbon phase separated in the settling operation is further processed, e.g., by fractionation, to recover the alkylate product and to separate unconsumed isoparaffin for recycle to the alkylation reactor.
It has been found necessary in prior art to maintain conditions in hydrogen fluoride catalyzed isoparaffin-olefin alkylation operations within fairly specific ranges. For example, conditions such as temperature, pressure, reactant and catalyst concentrations, etc., must be carefully regulated in order to provide an acceptable yield of high quality alkylate product. One of the alkylation conditions found to be essential in producing an alkylation reaction product of adequate quality has been the maintenance of a catalyst/hydrocarbon volume ratio above a minimum level in the alkylation reactor. As used herein, the term "catalyst/hydrocarbon volume ratio" means the ratio of the volume of catalyst introduced into a reactor or soaker per unit time divided by the volume of hydrocarbons, including hydrocarbonaceous alkylating agents such as alkyl fluorides, introduced into the reactor or soaker per unit time. It has been found necessary in prior art to maintain a catalyst/hydrocarbon volume ratio in the feeds to an alkylation reactor of at least about 1:1, and a catalyst/hydrocarbon volume ratio of about 1.5:1, or greater, is usually found to provide a product of superior quality. It has been found that, when lower catalyst/hydrocarbon volume ratios are utilized, so that the catalyst/hydrocarbon volume ratio is less than about 1:1 and often even when the catalyst/hydrocarbon volume ratio is less than 1.5:1, that the olefin concentration in the catalyst becomes relatively high, and results in a high rate of olefin polymerization in the alkylation reactor. It is well known in the art that olefin polymerization is a very undesirable side reaction, using up large amounts of valuable olefin feedstocks and producing low octane, overly high boiling product hydrocarbon. Efforst are normally made to avoid olefin polymerization in the alkylation reactor, if possible. Thus, it has been found essential in prior art to maintain the catalyst/hydrocarbon ratio in the alkylation reactor at a relatively high level, generally about 1:1 or more, in order to provide a high quality alkylate product and to avoid excessive consumption of olefinic feedstocks.
Another of the alkylation conditions found conducive to the production of high quality alkylate product in commercial operations has been the use in the alkylation reactor of a hydrogen fluoride alkylation catalyst having a relatively low acid strength (e.g., less than about 95 weight percent hydrogen fluoride and preferably between about 80 weight percent and about 90 weight percent hydrogen fluoride). Higher strength hydrogen fluoride alkylation catalyst may be used; however, the quality of the alkylate product produced using higher strength catalyst is significantly less than the quality of alkylate produced when lower strength catalyst, having the preferred 80-90 weight percent acid strength, is used in the alkylation reactor.
In a relatively recent modification, a reaction soaker has been added to the conventional alkylation scheme. The hydrocarbon reactants and hydrogen fluoride catalysts are first charged to a reactor-cooler equipped with heat exchange means and the reactants and catalyst are thoroughly admixed therein to form a reaction mixture. Within a short period of residence time, e.g., about 0.1-2 minutes, substantially all of the olefins charged to the reactor-cooler react with isoparaffin to form alkylate hydrocarbons with the simultaneous formation of large amounts of heat energy. This heat energy is removed from the reaction mixture in the reactor-cooler in order to maintain the reaction mixture at a fairly uniform temperature. After a short 0.1-2 minute residence time in the reactor-cooler, rather than simply settling the reaction mixture as in prior art, the reaction mixture is passed from the reactor into the reaction soaker which generally does not have heat exchange means. The reaction soaker is typically a relatively large vessel equipped with perforated trays, baffle sections or other means for maintaining the immiscible hydrocarbons and catalyst in the reaction mixture in the form of an emulsion. The reaction mixture is retained in the reaction soaker for a relatively long residence time, e.g., about 2-60 minutes. The reaction mixture is then removed from the reaction soaker and passed to a conventional settler for gravity separation into hydrocarbon and catalyst phases. It has been found that the use of a reaction soaker as described results in a substantial improvement in the quality and purity of the alkylate product in comparison with alkylation operations which employ only a conventional reactor or reactor-cooler. Use of the soaker has been found to provide a substantial increase in the motor and research octane ratings of the alkylate produced. This is believed to be due primarily to isomerization of relatively low octane alkylate hydrocarbons, such as dimethylhexane, within the soaker to form additional quantities of relatively high octane alkylate hydrocarbons, such as trimethylpentanes. Use of the reaction soaker has also been found to result in a reduction in the concentration of undesirable alkyl fluorides in the hydrocarbons treated in the reaction soaker, substantially eliminating the problem of fluoride contamination of the alkylate product.
Although use of the alkylation soaker as described above provides a substantial overall improvement relative to conventional alkylation operations, the long residence time of the reaction mixture within the soaker necessitates the use of a very large total inventory of hydrogen fluoride catalysts in the overall alkylation operation, with the major portion of the total inventory of catalyst in the overall system being located within the soaker at any given time. The relatively large amount of catalyst thus needed in the overall operation necessitates the use of larger size equipment in several sections of the alkylation system. The increased cost of maintaining this large catalyst inventory partially offsets the advantages obtained by using the reaction soaker. For example, every commercial alkylation operation generally includes a large catalyst storage drum which is of sufficient size to contain all of the catalyst used in the overall system when necessary. The catalyst inventory is stored in the drum during periods when the overall operation is shut down for any reason. When the catalyst inventory needed in an alkylation operation is substantially increased by the use of an alkylation soaker, the size of the catalyst storage drum must also be increased substantially. Further, the relief valve for catalyst storage drum and relief gas neutralization equipment must also be enlarged along with the catalyst storage drum. Heretofore, the beneficial results obtained using a reaction soaker in an alkylation operation have been, to some extent, hindered by the larger catalyst inventory thus required and the attendant increased investment and operating costs associated with the use of the soaker.
Other researchers have also been concerned with the problems caused by operating an HF alkylation zone with relatively low strength HF acid. Some researchers have thought that the problem was removal of alkyl fluorides from an alkylate. The solution proposed was to contact the alkylate in a contacting zone with HF acid of high purity. See U.S. Pat. Nos. 3,763,264 (Class 260.sup.1/2683.42) and 3,784,628 (Class 260-683.42), the teachings of which are incorporated by reference. In these patents, the patentee contacts the alkylate, containing alkyl fluorides, in a special contacting zone wherein the alkylate contacts high purity HF acid. In one patent, the HF acid is indicated as coming from the overhead of the HF acid rerun column and a separate HF fraction from the depropanizer column, in U.S. Pat. No. 3,763,264. Both of these HF acid sources would be relatively high purity, and both would be substantially free of any organic diluent, or acid soluble oil. In U.S. Pat. No. 3,784,628, the HF acid used is 98 wt. % pure, though the specific source of this acid is not mentioned.
In both of these mentioned patents, the patentee is concerned with removal of alkyl fluorides. If a refiner is only interested in removing alkyl fluorides, the very high acid purities suggested in these patents would be optimum. High acid strengths promote break down of alkyl fluorides into olefins and HF acid. Unfortunately, the patentees do not seem to appreciate the fate to be suffered by the olefins will very quickly react in these contacting zones, and the alkylate formed therefrom will be of a relatively low quality. The alkylate will be of low quality because the HF acid used does not contain any organic diluent or acid soluble oil, to attenuate the activity of the HF acid catalyst. The net effect of such an operation will be to increase slightly the end point of the gasoline, and slightly lower the octane number of the alkylate.
The process of this invention helps eliminate such problems and may be used to provide an alkylation operation having a substantially reduced catalyst inventory requirement while the high quality product obtained by employing a soaker is further improved.
Although better quality alkylate is produced when relatively low strength (80-90 weight percent) hydrogen fluoride catalyst is employed in the alkylation reactor, it has been found that improved results are obtained when a higher strength (e.g., greater than 90 weight percent, and preferably 90 to 95 weight percent) hydrogen fluoride catalyst is employed in an alkylation soaker. Thus, in an optimum system, lower strength hydrogen fluoride catalyst would be utilized in the alkylation reactor while higher strength hydrogen fluoride catalyst containing organic diluent would be utilized in the soaker. Heretofore, use of two distinct hydrogen fluoride catalyst phases, differing in acid strength, in an alkylation operation employing an alkylation soaker has not been possible, since the reaction mixture of hydrogen fluoride catalyst and hydrocarbons has been passed directly from the alkylation reactor-cooler into the soaker, the same hydrogen fluoride catalyst phase being used both is the reactor-cooler and the soaker. The process of the present invention provides an economical and convenient method for employing two hydrogen fluoride catalyst phases, differing in acid strength, in an alkylation operation using a soaker, while avoiding product degradation which would occur if an organic diluent free acid was used in a soaker.